The present invention relates to a semiconductor laser, and to a light source incorporating a semiconductor laser.
Due to their small size and low cost, semiconductor lasers are widely employed as light sources in fiber-optic communication systems, optical measurement systems, and the like. A typical semiconductor laser operates as a Fabry-Perot resonator, having cleaved facets at both ends that create an optical cavity in which lasing oscillation takes place. If the two facets are uncoated, light is emitted equally through both of them. Light emitted through the front facet is used for the intended application, e.g. communication or measurement. Light emitted through the rear facet is not used, or is used for monitoring the output power of the laser.
To increase the useful optical output of the laser, it is common to coat the end facets so that the front facet has a lower reflectivity than the rear facet. This increases the proportion of the optical power emitted through the front facet, but there is an unwanted consequence: more returning light, that has been reflected outside the laser, re-enters the laser cavity through the front facet. This returning light interferes with the operation of the laser, causing problems such as relative intensity noise and intermodulation distortion. Further details will be given later.